1. Field of the Invention
The present invention is directed to an electrical fuse and a method for its production having a metallically coated, partially etched fusible conductor welded or bonded onto contact surfaces in its housing.
2. Related Art
The welding or bonding offusible conductors onto contact surfaces in a fuse housing is especially preferred among fuses which are selected to be mounted onto printed circuit boards and the like by the so-called SMD-technique (surface mounted device technique). Conventionally, when mounting a fuse by the SMD-technique, the housing of the fuse is first glued to the printed circuit board. Then, solder for contacting the contact surfaces to the contacts of the printed circuit board and for mounting the fuse onto the printed circuit board is added with the aid of flow-soldering. Alternatively, when the solder is added by the melting of a soldering paste which has been applied earlier, no glueing of the housing is necessary. As a result of the soldering, the fusible conductor in the fuse housing may reach soldering temperatures. The heat can interrupt or otherwise compromise soldered connections in the housing for fixing the fusible conductor to contact surfaces in the housing. From the heat of soldering the housing to a printed circuit board also alloying by diffusion may take place whereby the characteristics of the fuse can be changed and at least an aging of the fuse occurs.
If the connection between the fusible conductor and the contact surfaces is instead accomplished by welding or bonding, these disadvantages resulting from applying heat during the mounting by soldering of the fuse housing onto a printed circuit board are prevented. However, welding or bonding of fusible conductors has its own disadvantages. When welding or bonding, a certain pressure on the fusible conductor and its respective contact surfaces is necessary whereby the fusible conductor and possibly the base material of the contact surfaces (or the plated contact surfaces) more or less deforms. These deformations can lead to cracks, especially micro-cracks, which may spread during loading in use. By each loading of the electrical fuse, the fusible conductor increases and decreases in length when a load is added and removed so that the point of welding or bonding is mechanically stressed by each switching on and off of a unit for which the fuse is used. Micro-cracks are especially difficult to detect initially in a new device because they are fully conductive at the outset and thus undetectable by electrical tests. Even optical tests using a microscope are no guaranty that damage will be detected.